The present invention relates generally to a spirally wound membrane filter element or cartridge for use in a standard cartridge filter housing together with a method for making such a cartridge, and more particularly, to a spirally wound cartridge, and method for making the same, in which the cartridge is tapered to provide a full fit with respect to the filter housing and to maintain relatively constant feed velocity over the membrane surface despite the passage of a portion of the feed through the membrane into the permeate tube.
Standard cartridge filter housings presently exist for receiving a filter cartridge. These standard cartridge filter housings are molded with a substantial draft angle or taper inside the sump portion of the housing to facilitate release of that part from the mold and, in some cases, to add strength to the sump. Such a filter housing comprises an elongated tapered sump portion having a closed bottom. If the housing is intended for use with a crossflow membrane filter element, it is also provided with a concentrate valve or orifice connected to the bottom. The housing also includes a housing cap through which the feed water or solution is introduced into the housing for exposure to the filter element and a permeate outlet for removing permeate or filtrate from the system.
Current technology with respect to the filter cartridges utilized in these cartridge housings ranges from standard pleated filter cartridges which utilize conventional "dead end" filtration technology and standard spirally wound membrane elements which utilize "cross flow" filtration technology. In "dead end" filtration technology there is no flow of feed water or solution relative to the membrane other than what passes through the filter media, while in "cross flow" filtration technology, the feed water, in addition to having a portion pass through the filter media, continually flows across the membrane surface and exits through a concentrate outlet port or valve. For purposes of this application, the term membrane is used to describe the filter material or media, whether it is technically a membrane or not. Conventional spirally wound membrane elements are generally cylindrical in shape and are formed by winding a relatively rectangular sheet of membrane material (together with appropriate spacer and permeate collection materials also relatively rectangular in shape) in a spiral configuration around a centrally disposed permeate tube or mandrel. This generally cylindrical filter cartridge is then disposed within the sump portion of the filter housing. However, because of the tapered configuration of the sump, the top portion of the filter cartridge is spaced inwardly from the interior surface of the upper end of the sump. A brine seal or concentrate seal is then positioned near the top of the filter cartridge between the outer surface of the cartridge and the inner wall of the sump. Such a seal is needed to force the feed solution, which is introduced at the top of the sump, to pass through the cartridge element flow channels defined by the spacer and over the membrane, and not around the outside between the sump wall and the outer surface of the filter cartridge.
While this construction is satisfactory in some applications, the provision of the brine or concentrate seal creates dead flow areas between the sump wall and the outer surface of the filter cartridge which can be havens for bacteria and other contaminants. Further, the above described structure requires the provision of such a brine or concentrate seal to operate sucessfully in the cross flow mode. This adds to the cost of the overall system, adds to the maintenance of it since such seal must be periodically cleaned, and limits its potential use since one more material with potential chemical incompatibility with the fluid is added to the filtration element. Further, existing spiral wound membrane cartridges lack uniform flow velocity over the entire membrane surface because of the fact that a portion of the feed water, during its passage through the membrane cartridge, is removed through the permeate outlet. Thus, during the passage of feed water or solution from one end of the cartridge to the other, the flow rate over the membrane surface decreases. This results in less predictable filter cartridge performance and shorter life of the membrane element due to the greater likelihood of cartridge plugging or blinding.
Accordingly, there is a need in the art for an improved filter cartridge element for use in a standard tapered filter housing, which cartridge overcome the problems and limitations currently existing in the art.